POSTNET is a barcode symbology that is used by the United States Postal Service to assist in directing mail. The ZIP Code or ZIP+4 code is encoded in this unique symbology that encodes data in half- and full-height bars. Most often, the delivery point is added, usually being the last two digits of the address or PO box number.

The barcode starts and ends with a full bar (often called a guard rail or frame bar and represented as the letter "S" in the USPS TrueType Font) and has a check digit after the ZIP or ZIP+4. The check digit is calculated as follows:

 1. Add up the digits, including the delivery point code if it is being used. (Zeroes are zero, not eleven.) If you are sending a letter to somewhere in Young America, Minnesota, you might be sending to 55555-1237, which would have the sum of 38

 2. Find the remainder of this number when it is divided by 10, in this case 8. This could also be called the sum modulo 10. (Note that when summing the digits, any leading 1 in the current sum can just be discarded.)

 3. Subtract the sum modulo 10 from 10. Continuing with the example, 10 - 8 = 2. The check digit is therefore 2.

 4. The sum of the ZIP, ZIP+4, or ZIP+4+delivery point digits and the check digit, modulo 10, should be zero. Continuing with the example, (5+5+5+5+5+1+2+3+7+2) mod 10 = 0.

There have been 4 formats of Postnet barcodes used by the Postal Service:

A 5 digit (plus check digit) barcode, containing the basic ZIP Code only, referred to as the "A" code. 32 bars total.

A 6 digit (plus check digit) barcode, containing the last 2 digits of the ZIP Code and the 4 digits of the ZIP+4 Code, referred to as a "B" code. Possibly 37 bars total. In the early stages of Postal automated mail processing the B code was used to "upgrade" mail that had been coded only with a 5-digit "A" code. This barcode was only found on mail that received a 5-digit barcode on the initial coding by an OCR. Now obsolete.

A 9 digit (plus check digit) barcode, containing the ZIP Code and ZIP+4 Code, referred to as the "C" code. 52 bars total. The 9-digit barcode enabled the sorting of mail to the individual delivery carrier, and in some cases into a semblance of delivery sequence.

An 11 digit (plus check digit) barcode, containing the ZIP Code, ZIP+4 Code, and the delivery point code. 62 bars total. This is usually referred to as the DPBC, or Delivery Point Bar Code. This is the predominant barcode in use currently (as of 2005), and it enables the Postal Service to sort mail into delivery point (address) sequence.